1. Field of the Invention
The present invention relates to an optical information recording/reproducing apparatus for optically recording Information on an optical card or other optical recording medium, and optically reproducing information existing on an optical recording medium. More particularly, this invention is concerned with an optical information recording/reproducing apparatus having a first light source for generating a recording light beam and a second light source for generating a reproducing light beam.
2. Description of the Related Art
In a dual light source type optical information recording/reproducing apparatus having both a light source for generating a recording light beam and a light source for generating a reproducing light beam, two light sources are installed Independently. Therefore, the apparatus enables so-called verification that uses a reproducing light beam to check if an immediately preceding recording is acceptable. Compared with a signal light source type apparatus in which spot light must scan a track of an optical recording medium twice In order to record data, the dual light source type apparatus permits a recording speed which is twice as fast. The dual light source apparatus can produce a focus servo signal and a tracking servo signal using a reproducing light beam whose emission is held constant. This results In stable servo control even during recording.
FIG. 1 shows an example of a construction of an optical system for an optical head incorporated in a dual light source type optical information recording/reproducing apparatus of the prior art. A recording light beam generated by a semiconductor laser 1 passes through a collimation lens 2 to become a substantially elliptic beam of parallel rays. The beam of parallel rays is trimmed by a trimmer prism, whereby the longitudinal axis of the ellipse is reduced to provide a circle. Then, the diameter of the trimmed beam of parallel rays is reduced by a circular diaphragm 4 so that the spot size of the recording light beam will be a given value. A circular beam emerging from the diaphragm 4 consists mainly of S-polarized components, which is characteristic of a semiconductor laser. Therefore, the circular beam is almost completely reflected from a reflector of a polarized beam splitter 5, and then converges on the optical axis of an objective 6. The incident light is condensed on an optical card 7 of the objective 6.
FIG. 2 shows a spot of a recording light beam formed on an optical card, and a reproducing light beam image. The recording light beam is condensed by an objective 6, and radiated as a circular light spot 23 on the optical card 7. The light spot 23, whose energy density is concentrated, causes the recording layer of the optical card 7 to show an irreversible thermal change, and forms a recording pit 22.
Multiple tracks 19 Including information recording tracks 20 and track guides 21 are formed on the optical card 7. During recording, the optical card 7 moves in the direction of an arrow a or b in FIG. 2 along the track guides 21. In this state, pulses modulated with information to be recorded are applied to the semiconductor laser 1. The semiconductor laser emits pulsed light in accordance with the modulation. With the emission of pulsed light, pits 22 are formed one after another on the optical card 7. Consequently, information is recorded as an array of pits on the information recording tracks 20.
In an optical head, a light-emitting diode 55 such as an end-face light-emitting diode having a slit type light-emitting surface is installed as a light source of a reproducing light beam. A reproducing light beam generated by the light-emitting diode 55 passes through a collimation lens 9 to become parallel light. Only the P-polarized components of the reproducing light beam are transmitted by the polarized beam splitter 5, and then, these P-polarized components enter a position decentered from the optical axis of the objective 6. Then, the incident light beam of the objective 6 produces, as shown in FIG. 2, an image 56 on the light-emitting surface of the light-emitting diode 55. Then, the image is formed on the optical card 7. The relative distance from the optical image 56 of the reproducing light beam to the recording optical spot 23 is adjusted as follows: when the optical head is assembled and adjusted, the optical axis of the recording light beam that has not yet entered the objective 6 and the optical axis of the reproducing light beam are aligned with each other with an angular difference between them.
The optical image 56 formed on the optical card 7 is reflected regularly from the optical card 7, wherein the quantity of light is modulated depending on the presence or absence of a track guide 21 and a pit 22. The reflected light passes through the objective 6 in the opposite direction, and heads for the polarized beam splitter 5 In the form of substantially parallel light. The reflected light still has P-polarized components because it results from regular reflection. Therefore, the reflected light is almost completely transmitted by the polarized beam splitter 5. The light transmitted by the beam splitter 5 is routed to a condenser lens 15 via a reflecting mirror 14. The light condensed by the condenser lens 15 is divided by a half-mirror 16. Beams of the divided light respectively enter a reproducing/tracking photodetector 17, and a focus photodetector 18 through light receiving surfaces thereof. Thereby, the optical image on the optical card 7 is magnified and projected.
The optical system in the optical head performs what is known as "off-axis focus detection." Specifically, the optical system radiates a reproducing light beam at a position decentered from the optical axis of the objective 6. Therefore, a half-split light-receiving element is placed on the focus photodetector 18 in order to detect movement of an image of a reproducing light beam resulting from deviation of focus.
FIG. 3 shows an optical image projected on a reproducing/tracking photodetector 17. Reproducing light-receiving elements 59 and 60, and tracking light-receiving elements 57 and 58 are placed on the photodetector 17. A magnified image 61 of an optical image 56 from a light-emitting diode 55 is projected on an optical card 7, and formed at an appropriate position on the light-receiving elements without track displacement or defocus. Each of the tracking light-receiving elements 57 and 58 detects a positional change of an image of a track guide 21 resulting from track displacement, as a change in the quantity of received light. A tracking error signal is generated using signals output by these light-receiving elements 57 and 58. During reproduction, each of the reproducing light-receiving elements 59 and 60 detects the presence or absence of a pit on each of two tracks as a change In the quantity of light, and outputs a regenerative signal representing the presence or absence of a pit.
Next, operations for recording will be described. A recording light beam modulated with a recording signal is radiated onto an optical card 7. A light spot 23 projected on the optical card 7 forms a pit 22. At this time, the card 7 is moving in the direction of an arrow a or b in FIG. 3 with respect to the light spot 23. For example, when the optical card 7 is moving in the arrow-a direction, the position of the pit formed 22 is moved toward an optical image 56 of a light-emitting diode. When the pit 22 reaches the optical image 56, the quantity of light in a reproducing light-receiving element 60 on a photodetector 17 changes due to the image of the pit 22. The reproducing light-receiving element 60 detects the change in the quantity of light and outputs a regenerative signal immediately after recording.
Therefore, when the optical card 7 moves in the arrow-a direction, a regenerative signal is produced immediately after recording. In the aforesaid construction, as long as the optical medium moves unidirectionally, verification can be carried out; that is, the acceptability of recording can be checked right away using a reproducing light beam.
However, the conventional dual light source type optical information recording/reproducing apparatus has a disadvantage which is described below. In the dual light source type apparatus, as shown in FIG. 2, a recording light beam spot and a reproducing light beam image are separated by a certain distance. During recording, tracking is done using light of the reproducing light beam reflected from an optical card. Therefore, when a track on the optical card is tilted or skewed relative to the direction of movement of the optical card, a pit which is formed with the recording light beam spot is decentered on a track.
Misregistration occurring when a recording medium is attached to an optical card substrate, poor precision in machining a card holding member used to drive an optical card in the direction In which tracks extend, and/or a split on a card member occurring during operation cause almost all the tracks on an optical card to be linearly skewed. Assuming that a recording light beam spot and a reproducing light beam image are positioned by 40 um apart on an optical card, and the skewing of tracks on the optical card amount to a maximum of 1 mm for the overall length of 85.6 mm of the optical card, the recording light beam spot is decentered by 0.47 .mu.m (=40 .mu.m.times.1/85.6) in the tracking direction.